It has long been recognized in the coin examining art that the interaction of an object with a low frequency electromagnetic field can be used to indicate, at least in part, the material composition of the object and thus whether or not the object is an acceptable coin and if acceptable its denomination. See, for example, U.S. Pat. No. 3,059,749. It has also been recognized that such low frequency tests are advantageously combined with one or more tests at a higher frequency. See, for example, U.S. Pat. No. 3,870,137 assigned to the assignee of the present application. The optimum methods for low frequency testing have, in the past, used bridge circuits which incorporate testing of both phase and amplitude effects of coin interaction with an electromagnetic field.
Another technique which has been popular in the testing of coins has been the transmit-receive technique in which an electromagnetic field is created by an inductor adjacent one face of a coin and characteristics of the received signal adjacent the other face are examined as a step in determining the coin's authenticity and denomination. For example, each of U.S. Pat. Nos. 3,599,771 and 3,741,363 discloses a transmitter coil creating an electromagnetic field at either end. Spaced adjacent each end of the transmitter coil is a secondary coil. The two secondary coils are electrically connected in series, and have opposing orientations with respect to the transmitting coil field. An unknown coin is placed between one secondary coil and the transmitting coil and a known coin is placed between the other secondary coil and the transmitting coil. The unknown coin is accepted only if the signal delivered by the secondary coils does not exceed a threshhold value. Such an arrangement, of course, is suitable only for examination of one coin denomination per testing station.
U.S. Pat. No. 3,966,034, assigned to the assignee of the present application, discloses a phase sensitive coin discrimination method and apparatus operating by the transmit-receive technique with particular utility in distinguishing between two similar coins such as the British 5 P and the West German 1 DM. Unlike the present invention, the detailed embodiments of that patent operate at relatively high frequencies (for example 320 kHz) and rely upon differences in coin volume to help distinguish between otherwise similar coins.
U.S. Pat. No. 4,086,527, discloses a transmit-receive type coin examining apparatus in which the transmitter coil is driven by a controlled variable frequency oscillator operated at one or more selected frequencies in the range of 5-300 kHz. The secondary or receiving coil is connected to an undisclosed "quantifying operator" circuit which obtains quantitative information regarding amplitude of the secondary signal and its phase with respect to the primary (transmitted) signal.
U.S. Pat. No. 4,398,626, assigned to the assignee of the present application discloses a transmit-receive type coin examination method and apparatus in which a nonlinear amplifier is employed between the receiving inductor and the phase shift measuring means in order to introduce an additional phase shift which is inversely related to the amplitude of the output of the receiving inductor. The additional phase shift improves the capability of the apparatus of the application to discriminate between various coins and particularly to discriminate between coins which produce nearly the same phase shift as measured by phase shift measuring circuitry lacking the nonlinear amplifier disclosed in U.S. Pat. No. 4,398,626.
European Patent Application No. 0 048 557, filed Sept. 2, 1981, discusses an electronic coin validator having a transmit coil and a receive coil for performing tests of coin face area and coin resistance. An automatic gain control circuit is described for use in modifying signal amplitude to provide compensation. This gain control circuit apparently has as its basic input the received signal amplitude for a transmitted signal having a frequency below 1 kHz. At least one absolute adjustment is needed to set up the validator in production.
Generally, low frequency test apparatus require at least one tuning element and at least one tuning adjustment during the manufacturing of such apparatus to compensate for components having slightly different values within tolerance and for variations in component positioning which occur during the construction of the test apparatus. For example, in low frequency coin test apparatus employing a bridge circuit, the bridge circuit is normally tuned to both the amplitude and the phase of the signal received when an acceptable coin is in the test position. An additional problem long recognized in the coin testing art is the problem of how to compensate for component aging, for changes in the environment of the coin apparatus such as temperature changes, and for similar disruptive variations which result in undesirable changes in the operating characteristics of the electronic circuits employed in coin test apparatus. Various discrete compensation circuits have been developed to meet this problem. See, for example, U.S. Application No. 308,548, filed Oct. 2, 1981 and assigned to the assignee of the present invention.